1. Field of the Invention
The present invention relates to a driving circuit for a vacuum fluorescent display.
2. Description of the Related Art
A vacuum fluorescent display (hereinafter, referred to as “VFD”) is a display device of a self-illuminating type for displaying a desired pattern by causing a direct-heating type cathode called a filament to emit thermoelectrons by causing the filament to generate heat by applying a voltage thereto in A vacuum chamber, and by causing the thermoelectrons to collide against fluorescent material on an anode (segment) electrode and causing it to illuminate, by accelerating the thermoelectrons using a grid electrode. VFDs have excellent features in terms of visibility, multi-coloring, a low operating voltage, reliability (environmental resistance) etc. and are utilized in various applications and fields such as cars, home appliances and consumer products.
For a VFD, as one scheme for applying a voltage to its filament, pulse-driving scheme has been proposed. Pulse-driving scheme is a scheme in which a pulse voltage (hereinafter, referred to as “filament pulse voltage”) generated by chopping a DC voltage relatively high compared to the ordinary nominal voltage of the filament is applied to the filament, and an illuminating state having a small intensity gradient etc. can be obtained.
FIG. 13 illustrates a conventional pulse-driving scheme. As shown in the figure, in the conventional pulse-driving scheme, a filament pulse voltage having a constant duty ratio is set in an external oscillator 30 or an external controller 40 based on a reference clock signal (an oscillation clock for the external oscillator 30 or a system clock for the external controller 40) and is continuously applied to a filament 11.
As an example of VFD driving circuit using a conventional driving scheme such as the external oscillator 30 or the external controller 40 (hereinafter, referred to as “conventional VFD driving circuit”), there is a technology disclosed in Japanese Patent Application Laid-open Publication No. 2002-108263.
Furthermore, in a conventional VFD driving circuit, a mechanism for adjusting the intensity of a VFD 10 is provided such that the VDF 10 can be displayed with proper intensity in response to the surrounding environmental conditions (such as the environmental illumination intensity) when operating the VFD 10. As mechanisms for adjusting the intensity of the VFD 10, a scheme called “grid dimming” in which the duty ratio of a voltage applied to a grid electrode 12 (hereinafter, referred to as “grid voltage”) is adjusted and a scheme called “anode dimming” in which the duty ratio of a voltage applied to a segment (anode) electrode 13 (hereinafter, referred to as “segment voltage”) is adjusted are commonly used. Hereinafter, the grid dimming and the anode dimming are collectively referred to as “dimming”.
Here, a conventional VFD driving circuit executes dimming based on a reference table of dimmer adjustment data and dimmer values as shown in FIG. 12 (a), for example. The dimmer adjustment data is data correlated to values that can be set as the duty ratios of the grid voltage and the segment voltage and are designated when dimming is executed by an external device to the VFD driving circuit. The dimmer adjustment data can be bit number of binary data in response to the resolution of dimming such as, for example, 10-bit binary data (DM0-DM9) in which DM0 shown in FIG. 12 (a) is the LSB (Least Significant Bit). On the other hand, the dimmer value is a value that can be set as the duty ratio described above, and can be defined as “pulse width TW/pulse cycle T” using the pulse width TW and the pulse cycle T shown in the wave form diagram in FIG. 12 (b).
== First Task ==
FIG. 14 shows a wave form diagram of the main signal in the case where dimming is executed such that the duty ratios of the grid voltage and the segment voltage are reduced by “½”, “¼”, “⅛”, in a state that a conventional VFD driving circuit keeps applying a filament pulse voltage having a constant duty ratio to the filament 11. The time period in which both of the grid voltage and the segment voltage shown in the figure are at a level H represents a time period that is at a voltage by which both of the grid electrode 12 and the segment electrode 13 are driven (hereinafter, referred to as “ON period”) and, during that time period, it is assumed that fluorescent material on the driven segment electrode 13 illuminates and a desired pattern is displayed on the VFD 10.
Here, during the ON period, in the time period in which the filament pulse voltage is at the level H, the intensity of the VFD 10 is reduced because the potential difference between the filament, and the grid electrode and the segment electrode becomes small. Furthermore, as shown in FIG. 14, the ON period becomes shorter and the rate of the time period in which the filament pulse voltage is at the level H increases in the ON period as the duty ratios of the grid voltage and the segment voltage are reduced. Therefore, the reduction of the intensity of the VFD 10 as described above becomes remarkable (It is said that the duty ratio, “⅛” of the grid voltage and the segment voltage is the minimum threshold).
That is, in the conventional VFD driving circuit, dimming is executed such that the duty ratios of the grid voltage and the segment voltage are reduced in order to reduce the intensity of the VFD 10. In this case, the rate of reduction in the intensity of the VFD 10 becomes larger, as the rate of the occupation in the time period in which the filament pulse voltage is at the level H receives influence to become larger in the ON period than that of the rate of the reduction in intensity of the VFD 10 based on the dimming. Therefore, for the conventional VFD driving circuit, it has been a challenge to execute desired intensity adjustment by dimming when the ON period is short.
== Second Task ==
In the conventional VFD driving circuit, the filament pulse voltage is designed to be applied to the filament at a constant duty ratio while fluctuation of the duty ratio is caused by variations and thermal properties of elements driving the filament and fluctuation of the filament power voltage etc. Furthermore, due to the fluctuation of the duty ratio, the effective value of the filament pulse voltage goes out of the tolerance defined for its nominal value (for example, nominal value±approximately 10%) and a problem has been raised, that degrading is caused for the intensity grade of the VFD display and the life of the VFD display is shortened due to degradation of the filament.
Then, in recent years, demand for improvement in further reliability has been increased for the VFD driving circuit. Therefore, in order to cope with the above problems, it is required to equip a mechanism for finely adjusting the duty ratio of the filament pulse voltage at an appropriate timing (improving the resolution). In the conventional VFD driving circuit, it is possible to improve the resolution relating to the adjustment of the duty ratio of the filament pulse voltage by setting high the frequency of the reference clock signal that is to set the filament pulse voltage.
However, in the conventional VFD driving circuit, the power consumption increases and, concurrently, noises interfering with apparatuses such as radio are generated when the frequency of the reference clock signal is set at a too high frequency in order to improve the resolution relating to the adjustment of the duty ratio of the filament voltage. On the other hand, when the frequency of the reference clock signal is set at a low frequency (the cycle is made longer), the frequency of the filament pulse voltage is also decreased. Thus, the frequency of the filament pulse voltage reaches within the audible band (generally, 20 kHz or lower) and sound noises are generated from the filament.
As described above, the method of adjusting the frequency of the reference clock signal, the problems described above might occur. Therefore, a new technique is sought for a mechanism for adjusting the duty ratio of the filament pulse voltage.